The present invention relates to an optical semiconductor device having an encapsulated optical semiconductor element, an optical communication device having the optical semiconductor device, and electronic equipment having the optical semiconductor device.
Conventionally, as an optical semiconductor device having an optical semiconductor element that requires light transmission, or optical penetration, such as a light emitting diode (LED) and a photodiode (PD), products produced by using the transfer molding of a light-permeable mold resin are widely used.
When a light-permeable mold resin is made of a single kind of resin mold material, there is an advantage that a particularly satisfactory light transmittance can be obtained, while such light-permeable mold resin has characteristics that its coefficient of thermal expansion is large. This raises a problem that a difference between the coefficient of linear expansion of the light-permeable mold resin and the coefficients of linear expansion of the optical semiconductor device, the lead frame and the bonding wire would cause disconnection of the bonding wire and occurrence of cracks in the package and the like in a high operating temperature range, as a consequence of which it becomes difficult to produce an optical semiconductor device of high reliability.
As a method for controlling the coefficient of thermal expansion of the light-permeable mold resin, adding filler to the resin mold material is known. Typical fillers include transparent inorganic powder of glass or the like.
FIG. 16 is a graph showing the temperature dependence of the refractive index of a filler material (glass) and the temperature dependence of the refractive index of a resin mold material (hereinafter referred to as a base resin mold material) to which no filler is added. In detail, FIG. 16 shows a graph showing the refractive indexes of the filler material and the base resin mold material in the form of relative values with the refractive index at a temperature of 25° C. assuming a value of one, wherein the filler material and the base resin mold material is adjusted such that a refractive index difference between them becomes zero at the temperature of 25° C. In FIG. 16, the filler material is silica glass, and the base resin mold material is an acid-anhydrous epoxy resin or a phenolic epoxy resin.
Also, in FIG. 16, the dashed line represents the temperature dependence of the refractive index of the filler material, and the solid line represents the temperature dependence of the refractive index of the base resin mold material.
As shown in FIG. 16, the refractive index of the base resin mold material exhibits a tendency of decrease as the temperature rises, while the refractive index of the filler material has no temperature dependence and exhibits a tendency of constancy.
FIG. 17 is a graph showing the temperature dependence of the light transmittance of the resin mold material when the filler is added to the base resin.
In FIG. 17, the light transmittance at 25° C. is assumed to have a value of one, and the light transmittances at other temperatures are represented by values relative to the light transmittance at 25° C.
In FIG. 17, the light transmittance depends on the refractive index difference between the base resin and the filler, and the light transmittance is one at the temperature at which the refractive index difference is zero. Then, the light transmittance exhibits a tendency of decrease as the refractive index difference is increased.
For the above reasons, since the refractive index difference is zero at 25° C. in the case of FIG. 17, the light transmittance at 25° C. is the highest, and the light transmittance exhibits a tendency of decrease as the temperature rises from 25° C.
FIG. 18 is a graph showing the temperature dependence of the optical output power of a general LED.
In FIG. 18, the optical output power at 25° C. is assumed to be one, and the optical output powers at other temperatures are represented by values relative to the output power at 25° C.
As shown in FIG. 18, the optical output power exhibits a tendency of decrease with the temperature rise. As shown in FIG. 18, the general LED, of which the optical output power has the tendency of decrease with the temperature rise, therefore has a problem that a decrease in the optical output power becomes significant at high temperatures and it is difficult to provide an optical semiconductor device having a constant optical output power at temperatures within the operating temperature range.
FIG. 19 is a graph showing a measurement result of the temperature dependence of the optical output power when the LED is encapsulated in a light-permeable resin to which no filler is added, and a measurement result of the temperature dependence of the optical output power when the LED is encapsulated in the light-permeable resin to which filler is added.
In FIG. 19, the solid line represents the temperature dependence of the optical output power in the case of encapsulation in the light-permeable resin to which filler is added, and the dashed line represents the temperature dependence of the optical output power in the case of encapsulation in the light-permeable resin to which no filler is added.
In FIG. 19, the optical output power at 25° C. is given a value of one, and relative value representation is provided.
As shown in FIG. 19, the optical output power of the LED when the filler is added has a larger fluctuation in the optical output power than when no filler is added. This is presumably attributed to a decrease in the light transmittance as a consequence of an increasing refractive index difference between the added filler and the base resin due to the temperature change.
Moreover, as shown in FIG. 19, when no filler is added, the fluctuation in the optical output power has a tendency that the fluctuation can be approximated by a linear equation (collinear approximation). When the filler is added, the fluctuation in the optical output power has a tendency that the fluctuation cannot be approximated by a linear equation. This means that a point at which the refractive index difference between the added filler and the base resin is zero (indicated by A in FIG. 19) is within the measurement temperature range.
As shown in FIG. 19, the conventional optical semiconductor device that uses the resin to which filler is added as an encapsulating resin has a problem that the characteristics of the optical semiconductor device are not stabilized within the operating temperature range.
JP 2003-3043 A discloses that epoxy resin is used as a base resin material and non-alkali glass filler is used as a glass filler material.
JP 2002-88223 A is another prior art reference that discloses a material obtained by adding filler to a resin mold material.
The reference discloses an epoxy resin to which an effect accelerating agent and inorganic filler are added. The reference also discloses that the epoxy resin has a high transparency under various temperature environments and is excellent in heat resistance, moisture resistance and low stressedness.
However, the epoxy resin, which has a high transparency under various temperature environments and is excellent in heat resistance, moisture resistance and low stressedness, has a problem that the light transmittance decreases as the temperature rises and the characteristics of the optical semiconductor device can hardly be stabilized at high temperatures, as may be understood from the fact that the transmittance decreases from 100 to 70 as the temperature rises from 25° C. to 100° C.
JP H05-25397 A also discloses a resin composition, of which the light transmittance reversibly varies with the temperature change and the coefficient of thermal expansion is small and which provides a hardened body that has excellent repetition durability.
However, there is a problem that it is unclear whether or not the resin composition is usable for the optical semiconductor device, i.e., whether or not the resin composition causes some trouble when it is used for the optical semiconductor device. There is a further problem that even if the resin composition can be used, the manufacturing method of an optical semiconductor device having the resin composition is not known.